In conventional tire construction, the carcass ply or plies are turned up around the bead cores and then extended radially outward in the sidewalls towards the tread in order to withstand the tensile stresses encountered during the use of the tire. As these stresses act in opposite direction on the carcass ply or plies proper and their upward turned portions, they cause shearing in the intermediate rubber.
Furthermore, the flexing of the sidewalls during travel subjects the carcass reinforcement and the upward-turned portions thereof to alternate stresses, in opposite directions, which causing additional shear stresses in this same zone. All of these stresses may lead to the rupture of the intermediate rubber and premature destruction of the tire.
To avoid these drawbacks, tire designers have extended the upward turned portions of the carcass plies into zones of lesser shear, that is to say a greater distance radially outward in the sidewalls; however, in such cases the rigidity of the sidewalls is increased; ruptures of the upward turned portions of the plies may take place by compression in the sidewalls and in any event the road behavior of the tire is greatly modified.
The desirability of minimizing or eliminating the extension of the turnup portions of the carcass ply (or plies) radially outwardly of the bead core is the premise on which prior art locked-beads designs were developed. The proposed advantages included improved bead durability, and reduced material costs. Furthermore, the locked-bead approach allows the sidewalls to have a reduced thickness in the vicinity of the bead region. This reduced sidewall thickness results in a better distribution of flexibility that is particularly advantageous with tires having a small height to width ratio (for example of the order of 0.6).
However, locked-bead tire designs require a careful consideration of how best to anchor the carcass plies in the bead region without an excessive concentration of stress at the carcass ply ends.
One approach is to clamp the carcass plies to the bead core. For example, U.S. Pat. No. 4,922,985. ('985) discloses a carcass ply having a main portion that extends between both head cores of the tire and turnup portions that are anchored around each bead core by a clamping member. According to '985, the clamping member comprises a strip of side-by-side cords of a heat shrinkable material embedded in a suitable elastomeric substance having a permanent thermal shrinkage of at least 2 percent.
Another approach, as disclosed by U.S. Pat. No. 4,185,676, ('676) is to extend the portions of the carcass reinforcement that have been turned upward around the bead rings toward the outside of the tire and embed them in an annular portion of the tire that protrudes from the outer lower portion of the corresponding sidewall. According to '676, a mechanical de-coupling is obtained between the elastic matrices surrounding the carcass reinforcement and its upward turned portions, thus eliminating shearing in these zones.
Another approach, as disclosed by WO95/23073 is, in each bead region of the tire, to wrap a turnup end of the carcass reinforcing ply around a wedge-shaped rubber section which is disposed adjacent and axially (or laterally) inward (towards the equatorial plane) of the bead.
It is continually the goal in the art to simplify the construction and reduce the expense of building locked-bead type tires, yet improve the durability, handling, rolling resistance and other properties of the tires.